This is radically different, Sonea and Panisset claimed, from evolution as described by Darwin, who focused on animals and plants. Animal and plant species, as well as other eukaryotic species, arise mainly by genetic isolation. Bacteria are never so isolated. Instead of tortoises and mockingbirds marooned on islands, mutating and adapting, diverging slowly into distinct subspecies and eventually new species, finally reaching a point where they can’t or won’t mate with other populations—instead of that, you have relentless bacterial togetherness.

